Data-transmission techniques entail putting the message into digital form. When the original data are of an analog rather than digital nature, they are first subjected to processing by being sampled, quantized and coded to obtain the required digital form. The message is then represented by a series of symbols selected from a two-element alphabet designated by the numbers "0" and "1". The message undergoes a binary coding operation in which an electrical signal is assigned to each of the "0" and "1" symbols, these signals being generally termed "low" and "high", respectively. Direct coding can be performed by using, for example, non-return-to-zero (NRZ) coding known as full-baud coding, or again diphase or Manchester coding. The digital modulating signal so produced is generally transmitted on a carrier by using modulations of the carrier which are of the same type as for an analog signal. At reception, the demodulation consists in determining which of the possible amplitude, frequency or phase states of the carrier has been transmitted. The demodulating or decoding arrangement is adapted to the kind of coding performed at the transmission end and the clock signal which was used to time the coding operation has to be reconstituted to allow the "1" and "0" symbols of the message to be extracted.
The most widely used demodulating techniques are coherent demodulation, where a local oscillator synchronized with the transmission oscillator provides a phase reference and demodulation is performed by multiplying the incident signal with this reference waveform, and differential demodulation where the received signal, delayed by the length of one binary digit or bit, is used as the phase reference. The demodulation decodes the transitions in the signal directly.
Coherent demodulation enables a better performance to be achieved than differential demodulation; to obtain a synchronous time base, a phase-locking loop is used. In certain techniques of this kind the loop includes circuits for digitally quantizing phase errors.
In another technique which is used for decoding and identifying errors, described in U.S. Pat. No. 3,335,224, the signal is sampled in a shift register at a rate which is a multiple of, and preferably twice, the bit rate. This technique is intended particularly for transition-coded digital diphase signals. The recorded samples corresponding to at least two successive bits are decoded by using gate circuits to produce the bit information and to identify errors which there may be due to transmission noise.